To start off, I by no means would wish to cause offense to the Jews by wearing such a devotional symbol without a meaning. Christianity is rooted within Judaism, the first "Christians" were Jews, and our moral teacher, Jesus (or Yeshua), was/is a Jew. While Christian, I am Gentile as I do not think my family comes from a Jewish line (at least in recent generations).
I would love to wear the star of David with the cross as a means of my support and solidarity for the Jewish peoples' faith and community, and of Israel, and with their very intimate and long history with God. I would love to one day, if possible, live in Israel. But it's with this same love that I wouldn't want to wear it if it did cause offense to them. I suppose the combination of the two is meant to symbolise Jewish roots within Christianity, the Old and New Testaments, or Covenants, sharing the same morals and values, being equal together etc. and I think Gentile Christians often forget the Jewish roots, and we ought to be sensitive and remember them.
So that would be the meaning behind me wearing it. I hope it would not be offensive to practicing orthodox Jews, even though I realise they may not feel that same way?
Edit: Thanks Alex, sorry it's my first time on Mi Yodeya, and my first time asking a question. I mean people who in particular do practice the Jewish faith, other than that I'm not sure how more specific I could be?